Since it is extremely difficult to predict occurrence of an earthquake, an emergency earthquake alert system is proposed in which earthquake information including earthquake occurrence time, seismic center location and earthquake size is calculated on the basis of earthquake observation information of earthquake P waves observed at a large number of earthquake observation points, and predicted arrival time predicted from the earthquake information and magnitude are informed to an area where earthquake S waves, which are main vibration, have not reached immediately after occurrence of an earthquake (Patent Document 1).
FIG. 7 is a block diagram illustrating this prior-art emergency earthquake alert system 100, in which a large number of seismographic networks 101-1, 101-2 . . . 101-n installed at earthquake observation areas throughout the country are connected to a seismic analysis processor through a common communication network, and if a receiving portion 102 of the seismic analysis processor receives a seismic waveform from any of the seismographic networks 101, an analysis portion 103 analyzes it, calculates three elements of earthquake (earthquake occurrence time, seismic center location, and earthquake size) and acquires a radius of felt area from a distance from the seismic center location and the earthquake size.
A first determination portion 104 determines whether earthquake information will be informed or not on the basis of information regarding whether the earthquake size is such that the seismic intensity scale of the Japan Meteorological Agency is a certain value or more at a specific location or the distance from the seismic center location is within a radius of felt area acquired by the analysis portion 103. Also, a prediction calculation portion 105 acquires standard intensity at a specific location form the distance from the seismic center location, the depth and the earthquake size and also acquires an amplification factor to be set from differences in the geological structure, building structure and the like at the specific location, and calculates earthquake parameters such as predicted intensity, maximum acceleration, predicted arrival time of earthquake S waves and the like at the specific location from the standard intensity and the amplification factor.
However, since the vibration detected by the seismographic networks 101 includes living noise such as lightning strike, construction work, traveling of vehicles and the like other than the earthquake waves, it is necessary to determine whether the detected vibration is vibration caused by earthquake P waves or vibration caused by living noise, and since determination requires predetermined time, calculation of the three elements of earthquake (earthquake occurrence time, seismic center location, and earthquake size) is delayed in the end and an alert for a near-field earthquake could not be made in time.
Thus, an alert system (Patent Document 2) in which a plurality of seismometers are installed around a specific area, and if the intensity of S waves estimated from the maximum value of a speed in a perpendicular direction of the earthquake P waves detected by any of the seismometers exceeds a set level, an alert is issued to the specific area or a seismographic data collection system (Patent Document 3) in which observation data obtained when a plurality of observatories observe a seismic motion having the size larger than a given setting is collected by a data collection center, and required information such as seismic intensity immediately after occurrence of an earthquake is obtained are proposed.
However, in the above-described method in which it is determined to be occurrence of an earthquake if earthquake intensity detected by a seismometer exceeds a set level, even if vibration caused by a living noise exceeds the set level, it is erroneously determined to be an earthquake, and an earthquake alert is not sufficiently reliable. As a method to solve this, an alert cancellation method (Patent Document 4) is known in which an alert issued upon a first break of P waves is cancelled by verifying the alert when S waves arrive and if the alert is found to be unnecessary.